Methods and appratus for preparation of three-dimensional bodies

ABSTRACT

Method and apparatus for producing objects from fibrous monolith composites are provided. Two- and three-dimensional objects can be prepared from a continuous fibrous monolith filament. The objects can have complex geometries.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on, and claims the benefit of,co-pending U.S. Provisional Application Serial No. 60/251,171, filed onDec. 4, 2000, and entitled “Solid Freeform Fabrication Method forFibrous Monolith Composites.”

[0002] The present invention was made with U.S. Government support undergrant Number DE-FC02-96CH10861, awarded by the Department of Energy, andunder grant Number NAS8-00081 awarded by the National Aeronautics andSpace Administration. Accordingly, the Government may have certainrights in the invention described herein.

FIELD OF THE INVENTION

[0003] The present invention relates to processes for the fabrication oftwo- or three-dimensional fibrous monolith (FM) composites. Theinvention allows articles having complex geometries to be fabricatedfrom a single continuous fibrous monolith filament while controllingseveral critical processing parameters.

BACKGROUND OF INVENTION

[0004] Fibrous monoliths (“FMs”) are a unique class of structuralceramics that have mechanical properties similar to continuous fiberreinforced ceramic composites (CFCCs). Such properties includerelatively high fracture energies, damage tolerance, and gracefulfailures. In contrast to CFCCs, FMs can be produced at a significantlylower cost. FMs, which are monolithic ceramics, generally aremanufactured by powder processing techniques using inexpensive rawmaterials. As a result of the high performance characteristics of FMsand the low costs associated with manufacture of FMs, FMs are used in awider range of applications than heretofore typical for ceramiccomposites.

[0005] In preparing FM composites, starting materials typically areformed into filaments having predetermined lengths. These FM greenfilaments can be wound around a drum or mandrel as they are prepared toprovide a desired object, or prototype, upon removal form the drum ormandrel. Other methods of forming the objects include molding, cuttingand machining. Thus, in the fabrication of FM composite materials andobjects, the working of the individual filaments is a labor intensiveand time-consuming process.

[0006] There remains a need for a less labor-intensive process forpreparing prototypes from FM composites that permits the fabrication ofFM structures having complex geometries. There also remains a need for aprocess for preparing prototypes from FM composites to increase theproduction rate, reproducibility and quality of FM composite parts.

SUMMARY OF THE INVENTION

[0007] The present invention overcomes the disadvantages of priorfabrication methods. In the present invention, methods are provided forforming stronger, more durable FM prototypes. More specifically, anautomated process utilizing a high pressure extruder head, which may bemechanically controlled, is utilized to extrude a continuous FM filamentonto a surface, which may be mechanically controlled, to formthree-dimensional objects.

[0008] In one embodiment, a computer-controlled high-pressure extrusionhead with a 4-axis computer numerically controlled (CNC) motorized stageprovides for extrusion and deposition of FM components. The processresults in the production of solid objects directly from a computermodel without part-specific tooling or human intervention. The SFFprocess permits the manufacture of larger, more complex parts withintighter tolerances.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective cross-sectional view of a uniaxial fibrousmonolith composite in accordance with the present invention;

[0010]FIG. 2 is a schematic flow diagram showing a process of preparingfilaments in accordance with the present invention;

[0011]FIG. 3 is a block flow diagram showing a process for furtherpreparing the filaments of FIG. 2;

[0012]FIG. 4 is a photomicrograph of an axial cross-section of an FMcomposite prepared by the process of FIG. 3;

[0013]FIG. 5 is a photomicrograph of an axial cross-section of a secondFM composite prepared by the process of FIG. 3;

[0014]FIG. 6 is a schematic illustration of an apparatus of the processof FIG. 3, showing extrusion of a plurality of FM filaments;

[0015]FIG. 7 is a perspective view of the apparatus of FIG. 6;

[0016]FIG. 8 is a cross-sectional end view of the apparatus of FIG. 7;

[0017]FIG. 9 is a front elevational view of the apparatus of FIG. 7;

[0018]FIG. 10 is a front elevational view of the apparatus of FIG. 7;and

[0019]FIG. 11 is a front elevational view of a second apparatus inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention allows for the preparation of a widevariety of objects from FM materials. The process includes pressing afeed rod of the compounded engineering thermoplastic raw material. Thefeed rod is loaded into heated barrel to provide a molten material. Themolten material directly feeds into a fine deposition nozzle whosevolumetric flow rate can be adjusted for high raw material dispensing.The molten thermoplastic is extruded through the deposition nozzle ontoa surface, such as a foam pad. The surface is mounted on a 4-axis,motorized, computer numerically controlled (CNC) platen.

[0021] The invention utilizes a software program that allows for formingof an entire composite object out of one single, continuous fiber.Laminations of ‘green’ parts by warm isostatic pressing process allowfor objects having essentially no porosity with a surface finish of the12-S quality.

[0022] A. Filament Preparation

[0023] As used herein, “fibrous monolithic composite” and “fibrousmonolith” are intended to mean a ceramic and/or metallic compositematerial that includes a plurality of monolithic fibers, or filaments,each having at least a cell phase surrounded by a boundary phase but mayinclude more than one core and/or shell phase. Fibrous monoliths exhibitthe characteristic of non-brittle fracture, such that they provide fornon-catastrophic failure.

[0024] As used herein, “cell phase” is intended to mean a centrallylocated primary material of the monolithic fiber, that is dense,relatively hard and/or strong. The cell phase extends axially throughthe length of the fiber, and, when the fiber is viewed in transversecross-section, the cell phase forms the core of the fiber. The “cellphase” also may be referred to as a “cell” or “core”.

[0025] As used herein, “boundary phase” is intended to mean a moreductile and/or weaker material that surrounds the cell phase of amonolithic fiber in a relatively thin layer and that is disposed betweenthe various individual cell phases, forming a separating layer betweenthe cell phase and surrounding cell phases when a plurality of fibersare formed in a fibrous monolithic composite. The “boundary phase” alsomay be referred to as a “shell,” “cell boundary” or “boundary”.

[0026] Fibrous monoliths (“FMs”) are a unique class of structuralceramics that have mechanical properties similar to continuous fiberreinforced ceramic composites (CFCCs). Such properties includerelatively high fracture energies, damage tolerance, and gracefulfailures. In contrast to CFCCs, FMs can be produced at a significantlylower cost. FMs, which are monolithic ceramics, generally aremanufactured by powder processing techniques using inexpensive rawmaterials. As a result of the high performance characteristics of FMsand the low costs associated with manufacture of FMs, FMs are used in awider range of applications than heretofore typical for ceramiccomposites.

[0027] As shown in FIG. 1, the macroarchitecture of an FM composite 10generally includes multiple filaments 12 each comprising at least twodistinct materials—a primary phase in the form of elongatedpolycrystalline cells 14 separated by a thin secondary phase in the formof cell boundaries 16. Typical volume fractions of the two phases arebetween about 50 to about 99% of the fiber for the primary phase(polycrystalline cell) and between about 1 to about 50% of the fiber forthe interpenetrating phase (cell boundary). Preferably, the volumefractions are between about 80 to about 95% for the primary phase(polycrystalline cell) and between about 5 to about 20% for theinterpenetrating phase (cell boundary). The primary or cell phasetypically consists of a structural material of a metal, metal alloy,carbide, nitride, boride, oxide, phosphate or silicide and combinationthereof. The cells are individually surrounded and separated by cellboundaries of a tailored secondary phase. Powders that may be used inthe secondary phase include compounds to create weak interfaces such asfluoromica, and lanthanum phosphate; compounds to create porosity in alayer which function to create a weak interface; graphite powders andgraphite-containing powder mixtures; and hexagonal boron nitride powderand boron nitride-containing powder mixtures. If a metallic debond phaseis desired, reducible oxides of metals may be used, e.g., nickel andiron oxides, or powders of metals, e.g., nickel, iron, cobalt, tungsten,aluminum, niobium, silver, rhenium, chromium, or their alloys.

[0028] Advantageously, powders which may be used in the cell and/orboundary phase composition to provide the green matrix filament includediamond, graphite, ceramic oxides, ceramic carbides, ceramic nitrides,ceramic borides, ceramic silicides, metals, and intermetallics.Preferred powders for use in that composition include aluminum oxides,barium oxides, beryllium oxides, calcium oxides, cobalt oxides, chromiumoxides, dysprosium oxides and other rare earth oxides, hafnium oxides,lanthanum oxides, magnesium oxides, manganese oxides, niobium oxides,nickel oxides, tin oxides, aluminum phosphate, yttrium phosphate, leadoxides, lead titanate, lead zirconate, silicon oxides and silicates,thorium oxides, titanium oxides and titanates, uranium oxides, yttriumoxides, yttrium aluminate, zirconium oxides and their alloys; boroncarbides, iron carbides, hafnium carbides, molybdenum carbides, siliconcarbides, tantalum carbides, titanium carbides, uranium carbides,tungsten carbides, zirconium carbides; aluminum nitrides, cubic boronnitrides, hexagonal boron nitrides, hafnium nitride, silicon nitrides,titanium nitrides, uranium nitrides, yttrium nitrides, zirconiumnitrides; aluminum boride, hafnium boride, molybdenum boride, titaniumboride, zirconium boride; molybdenum disilicide; lithium and otheralkali metals and their alloys; magnesium and other alkali earth metalsand their alloys; titanium, iron, nickel, chromium, cobalt, molybdenum,tungsten, hafnium, rhenium, rhodium, niobium, tantalum, iridium,platinum, zirconium, palladium and other transition metals and theiralloys; cerium, ytterbium and other rare earth metals and their alloys;aluminum; carbon; lead; tin; and silicon.

[0029] Compositions comprising the cell phase differ from thosecomprising the boundary phase in order to provide the benefits generallyassociated with FMs. For example, the compositions may includeformulations of different compounds (e.g., HfC for the cell phase andWRe for the boundary phase or WC—Co and W—Ni—Fe) or formulations of thesame compounds but in different amounts (e.g., WC-3% Co for the cellphase and WC-6% Co for the boundary phase) as long as the overallproperties of the compositions are not the same. For example, thecompositions can be selected so that no excessively strong bondingoccurs between the two phases.

[0030] The cell boundary phase may be selected to create pressure zones,microcrack zones, ductile-phase zones, or weak debond-type interfaces inorder to increase the toughness of the composite. For example,low-shear-strength materials such as graphite and hexagonal boronnitride make excellent weak debond-type cell boundaries and are presentin Si₃N₄/BN and SiC/Graphite FM composites. The weak BN and graphiteinterfaces deflect cracks and delaminate thereby preventing brittlefailure of these composites and increasing their fracture toughness. Asa result, FM structures exhibit fracture behavior similar to CFCCs, suchas C/C and SiC/SiC composites, including the ability to fail in anon-catastrophic manner.

[0031] Various methods of preparing fibrous monolithic filaments areknown in the art, including the methods disclosed in U.S. Pat. No.5,645,781, which is incorporated by reference herein in its entirety.Generally, as illustrated in FIG. 3, the process of preparing fibrousmonolithic filaments in accordance with the present invention includesseparately blending the starting materials for a core 20 and shell 22,forming the core 24 having a first composition and forming the shell 26having a second composition, forming the feed rod 28 from the core andshell, and extruding the feed rod 30 one or more times to provide aceramic filament 32. The filaments may then be formed and/or arranged toprovide the desired structure in accordance with the present invention.

[0032] Fibrous monolith composites are fabricated using commerciallyavailable ceramic and metal powders using a process for convertingordinary ceramic powder into a “green” fiber that include the powder, athermoplastic polymer binder and other processing aids. The fiber iscompacted into the “green” state to create the fabric of elongatedpolycrystalline cells that resemble a fiber after sintering or hotpressing. The process is widely applicable, and allows a cell/cellboundary bi-component fiber to be made from a thermodynamicallycompatible set of materials available as sinterable powders. The scaleof the microstructure is determined by the green fiber diameter (cellsize) and coating thickness (cell boundary). Once the green compositefiber is prepared, it can be formed into a desired object using themethods of the present invention.

[0033] The core and shell of the feed rod are formed of mechanicallyactivated and agglomerate-free powders. The powders, such as the metals,alloys, carbides, nitrides, borides, oxides and silicides listed above,are selected to provide the desired mechanical properties in the finalcomposite. A wide variety of combinations of powders may be used for thecore and shell materials. Powders having particle size distributions inthe range of about 0.01 to about 100 microns (μm) in size may be used.Preferably, the particle size of the powder is between about 1 to about10 microns.

[0034] Filaments having more than one cell composition and/or more thanone shell composition can also be prepared to provide the benefits ofthe properties of the additional composition and/or to insulate theshell material. As an example, a layer of a second cell composition maybe disposed around the shell, such that the filament includes a centralcell, an intermediate shell and an outer cell. Other combinations ofcells and shells also may be prepared as desired. For example, a corematerial in combination with a plurality of different shells may beused.

[0035] A plurality of filaments may be bundled together and disposedwithin another shell. This arrangement of filaments results inessentially a “honeycomb” configuration when arranged to form the FMcomposite, as shown in FIG. 4. The bundled arrangement maintains themechanical behavior of the filaments but insulates a “weaker” shellmaterial from the external environment and any harsh conditions.

[0036] Numerous modifications and adjustments to the process forpreparing filaments may be made to allow for variations in theparticular compositions used to prepare the filaments. For example,viscosities may be adjusted, the diameter of the extrusion die may bechanged, or the relative volumes of the core and shell may be changed.Other methods for extruding the filaments known to those of skill in theart also may be utilized in combination with the processes and apparatusof the invention. For example, any modified process for continuousco-extrusion may be used.

[0037] Although the invention is described with reference to generallycylindrical-shaped FM filaments that are bundled together to form FMcomposites wherein the shape of the filaments become essentiallyhexagonal in cross-section as a result of processing, otherconfigurations are contemplated, as will be appreciated by those skilledin the art. For example, filaments having square, rectangular ortriangular cross-sections may be obtained by varying the shape of theextrusion die accordingly. Additionally, the shape of the die used inthe laminating step also may be modified accordingly as desired. Thus,different shapes and configurations of filaments in the FM composite maybe obtained, which may impact the resultant mechanical properties of theFM composite.

[0038] A binder burnout furnace, such as commercially available fromLindberg, Watertown, Wis. is used to remove polymer binder from theformed composite coatings and FM composite structures. Sinteringprocesses, including hot pressing, hot isostatic pressing orpressureless sintering, provide final consolidation and densification ofthe composite coatings and FM composite structures. A typical inductionhot-press such as commercially available from Vacuum Industries,Somerville, Mass. is capable of a maximum temperature of 2400° C. and amaximum load of 100 tons and can be operated in several differentenvironments including vacuum, argon, and nitrogen atmospheres.

[0039] B. Filament Deposition Process

[0040] Referring now to FIG. 3, the process of the present inventiongenerally includes preparation of FM feed rods 20 and further processingof the feed rods in an apparatus that includes an extrusion mechanism 22for extruding 24 the filament and a platen 26 onto which the extrudedfilament is deposited 28 in forming the desired object 30. The extrusionmechanism and/or the platen may be mechanically controlled 42, 44 usinga software program 40 in order to direct the filament as it is depositedonto the platen. The extrusion mechanism includes a gantry for theextrusion head, a motorized 4-axis stage, and a motion control system.

[0041] Multifilament feed rod 10 also may be used, and the feed rodspassed to the extrusion mechanism 22, as illustrated in FIG. 6. Themultifilament feed rods are prepared by bundling a plurality of FMfilaments of predetermined length, encasing the bundle within an outershell, if desired, and pressing the filaments into a feed rod.Additional volume reduction of the FM filaments is provided as a resultof the FM filaments being extruded twice, once during preparation of thefilaments from the starting materials and again during theextrusion/formation process, particularly where a plug of bundledfilaments is utilized. Additionally, multifilament feed rods allowfilaments having smaller cross-sectional diameters to be prepared.

[0042] The extruded filament is deposited on a surface of the platenlocated below the extruder head. The FM filament is deposited to formthe desired shape of an object, with the axis of filament parallel beingdeposited generally parallel to the surface of the platen. The extruderhead and/or the platen may be mechanically controlled to direct theextruded filament into the desired shape.

[0043] 1. Motion Architect Program to Fabricate “Green” FM Components

[0044] The process of the present invention utilizes a motion architectprogram for building FM composite parts. For the design and theoptimization of the motion architect program, ‘green’ 3 mm×4 mm×55 mmtest bars and 3″×3″ coupons and a thruster of complex geometry werebuilt. Consolidation parameters, which resulted in fully dense parts byhot pressing, were established. Hot isostatic press (HIP) parameterswere also determined. Room and elevated temperature flexure and tensiletesting evaluated the thermo-mechanical properties of the coupons andtest bars.

[0045] In this example, the motion architect program was designed forbuilding a ‘green’ rocket thruster. Two methods were evaluated to createthe architecture program. First, a commercially available computersoftware program was used to generate the architect program for therocket thruster. The commercial software program separates the drawingof the rocket thruster into individual slices and further into straightlines. From there, the X, Y, and Z locations for the extrusion areextracted. The software program outputs code in a machine language whichis then stripped of all machine specific commands, leaving the basiccoordinates as a base for a motion architect language program—thelanguage of the SFF machine. The scale between one machine-unit and oneSFF-unit is determined and introduced into the motion architect program.The relation between one machine-unit and one SFF unit is 20:1. Also,SFF specific commands are introduced to control the parameters asdescribed in Table 1. TABLE 1 Parameter Description Start delay The lagtime between layers Pre-flow Extrusion rate during stops Start flowExtrusion rate at the start of the exit distance Start distance Distancefrom start point for main flow to start Main flow Extrusion rate overthe bulk of the layer Shutoff distance The distance from the stop pointfor main flow to stop Rollback The rate the material is pulled back toreduce the flow Speed The rate at which the head moves along a set pathAcceleration The rate at which the extrusion head changes speed

[0046] In addition, various other parameters were optimized to obtain afully dense part that is within dimensional tolerances with anacceptable surface finish. These parameters include the thickness of theslices, lateral distance of the filament during the extrusion process,path velocity, extrusion speed, and the extrusion nozzle diameter andgeometry. Known literature can serve as a useful guide to understandingthe influence of the different parameters on the finished object toassist with optimizing operating parameters.

[0047] The first approach of using a commercially available softwareprogram to generate the program did not deliver optimal results. Optimalresults were not achieved because typical commercially availablesoftware programs were capable of producing only straight moves. Mostmotion architecture programs typically have very complex features andtend to be very lengthy. Consequently, large amounts of individualsegments must be downloaded from the personal computer to the motioncontroller. Because of the large amount of downloading required, themotion of the nozzle stopped after each segment, but extrusion of thematerial continued. As a result, heaps of excess material formed andbulges in the finished part occurred. The extrusion nozzle must be keptmoving. To keep the nozzle moving, the parameters of Table 1 wereintroduced.

[0048] A second approach to creating the motion architecture program wasutilized. The program was written line by line. Writing the program lineby line results in a shortened code since precompiled routines are used.Consequently, the parameters of Table 1. were eliminated in the new codemaking it possible to build an entire part out of one single fiber. Todetermine the ideal path velocity, extrusion speed, lateral distancebetween paths and slice thickness, samples of cylinders were extruded.

[0049] 2. Optimization of Operating Parameters

[0050] a. Test Billet Production

[0051] Initially, three 3″×3″ billets were produced and tested. Table 2lists the fabrication parameters and the material properties of theconsolidated billets. The flexural strength of the uniaxial billetsbuilt by traditional hand layup was 60% of the strength of monolithicZrC (60 ksi) and was typical of FM composites. However, the test barsfailed catastrophically with no load retention after initial failurethat is atypical of FM composites. An explanation for this behavior wasfound in the billets. Within which, upon examination, several smallsurface cracks were distinguishable. These cracks correspond to areas inthe billet where the W/Re interface was not uniformly distributed anddiscontinuous. A discontinuous WRe interface can result in poorcomposite behavior during testing. The viscosity of theceramic/thermoplastic and metal/thermoplastic mixtures by reformulatingthe recipes. TABLE 2 Hot Press Run 489/Billet Material Characteristics 12 3 Material ZrC(10%SiC)/WRe-HfC ZrC(10%SiC)/WRe-HfC ZrC(10%SiC)/WRe-HfC Architecture Uniaxial MFCX* 12 Layers Uniaxial SFCX 16 LayersBiaxial MFCX* 12 Layers Layup Hand Layup SFF Hand Layup Laminating DataGreen Weight 214.00 g 202.00 g 238.42 g Laminating Temperature 140° C.140° C. 140° C. Laminating Pressure 30,000 psi 30,000 psi 30,000 psi HotPressing Set Point 1950° C. 1950° C. 1950° C. Atmosphere N₂ N₂ N₂ SoakTime 1 hr 1 hr 1 hr Pressure 4000 psi 4000 psi 4000 psi MaterialProperties Flexural Strength (ksi) 33.91 9.62 26.28

[0052] The first rapid prototyped billet exhibited low flexural strengthin comparison to the billets built by hand. However, theload-displacement curve of one of the test bars revealed FM compositebehavior in failure. Optimization of the SFF extrusion parameters andviscosity matching of the ZrC core/WRe interface lead to improvedstrength and composite behavior of the billet and components fabricatedby this process.

[0053] b. Optimization of ZrC and WRe Recipes

[0054] As described above, FM feed rods are two component systemsconsisting of a primary core material and a secondary shell material.When individually mixing these components in preparation forco-extrusion, the torque on a C. W. Brabender Plasti-Corder TorqueRheometer is measured. Typically, the torque measurement of the corematerial must be between 10% to 20% higher than the shell material. Whenthe torque of the core material is not between 10% to 20% higher thanthe shell material, a uniform co-extruded filament will not be obtained.

[0055] Using a conventional extruder, no filament extrusion problemswere observed with the initial ZrC/thermoplastic and WRe/thermoplasticrecipes. However, when the same recipes were used with the apparatus andmethod described herein, various problems with the extrusion, wereobserved, including no shell material being extruded, inconsistentextrusions where the shell material is extruded in waves, and extrusionswhere the shell material blends into the core material. The formulationsfor the filaments were adjusted to address such problems. Severaliterations led to the recipes and the improved formulations listed inTables 3 and 4. In the ZrC recipe, Luwax, a hydrocarbon wax used in theold recipe, was replaced by the higher melt flow index EEA binder toreduce the viscosity of the core material. This required a similaradjustment to the WRe interface material to achieve viscosity matchingduring co-extrusion. TABLE 3 New ZrC-Recipe Type ZrC ‘Core’ materialBatch Size 42 Cc Batching Temperature 150  Deg C Batching Speed 60 RpmMaterial Density (g/cc) Volume % Volume (cc) weight (g) ZrC (10% SiC)6.350 48.00 20.16 128.02 EEA (MFI 20) 0.930 21.00 8.82 8.20 EVA 0.94021.00 8.82 8.29 Butyl Oleate 0.873 10.00 4.2 3.67 3.528 100.00 42.00148.18

[0056] TABLE 4 New Wre-Recipe Type WRe ‘Shell’ material Batch Size 42 CcBatching Temperature 150  Deg C Batching Speed 60 Rpm Material Density(g/cc) Volume % Volume (cc) Weight (g) W,Re,HfC 19.300 52.63 22.10426.62 EEA (MFI 20) 0.930 27.37 11.50 10.69 EEA (MFI 1.5) 0.930 20.008.40 7.81 100.00 42.00 445.12

[0057] c. Nozzle Geometry

[0058] A modified nozzle geometry is necessary to facilitate thematerial flow through the heating chamber and the extrusion compartment.The nozzle apparatus is shown in FIGS. 6-10. Generally, the nozzle 100includes a feed chamber 102 having a frustoconical portion 101 intowhich the filament feed rod is initially fed. The chamber 102 feeds intoan extrusion chamber 104 having a predetermined cross-sectional area forextruding the filaments. A nozzle tip (not shown) is attached to the end106 of the extrusion chamber 104 opposite the chamber 102. The filamentis fed from the nozzle tip onto the surface of the platen.

[0059] Another embodiment of a nozzle apparatus 200 is shown in FIG. 11.Among other differences, the feed chamber 202 is longer and theextrusion chamber 204 is shorter than those of the nozzle apparatus ofFIGS. 7-10.

[0060] d. Build Speed

[0061] The build speed does not have a significant influence on theoutcome of the finished “green” part. Tests were run at 6000, 4000, and1000 steps per minute. All the parts were acceptable from an extrusionpoint of view. However, at 6000 steps per minute some steps are lostbecause of the inertia of the machine, slow electronics, and otherspecific characteristics of this system such as running withoutfeedback. As result, distortion of the part occurs. Consequently, abuild speed of 4000 steps/minute is preferred.

[0062] e. Extrusion Speed

[0063] Cylinders were extruded to determine the ideal extrusion speedand slice thickness. One cylinder was extruded with an extrusion speedof 0.32 times the path velocity; slice thickness was 0.013″. A bulge inthe resultant cylinder indicated that the extrusion speed was too highand the slice thickness too small. A second sample was extruded at 0.2and 0.016″ slice thickness. Visible irregularities occurred, indicatingthat there was not enough material extruded, causing voids and thereforeirregularities in the part. Refining software code and adapting theextrusion nozzle geometry and diameter can eliminate thesediscrepancies.

[0064] A third sample was acceptable. The sample was extruded at 0.25and the slice thickness was 0.016″. Measurements of this same arepresented in Table 5. TABLE 5 Dimension Theoretical Actual Wallthickness 0.032″ 0.053″ Height 0.280″ 0.277″ Outer diameter 0.500″0.545″ Inner diameter 0.468″ 0.400″

[0065] f. Extrusion Temperature

[0066] The preferred extrusion temperature is 143° C. One test was runat 130° C. resulting in high extrusion pressures and virtually noimprovement in the lay up of the part. In a second test, the temperaturewas set to 155° C., but this test also yielded poor results and noimprovement in the part's extrusion. High extrusion temperatures andhigh extrusion pressure expose the equipment to unnecessary strain,therefore 143° C. appears to be the preferred extrusion temperature forthis material/binder system.

[0067] g. Minimize Pore Volume

[0068] Increasing the extrusion speed results in reduced pore volumes.The extruded material presses into the open space between the filamentsand reduces the pore volume between the extruded filaments. However, asseen in FIGS. 4 and 5, some residual porosity between the filamentsresults even with the increased extrusion speed. Warm isostatic pressing(WIP) process eliminates virtually all of the pores, thereby improvingthe consolidating process and facilitating the handling of the parts inthe green and brown state of the manufacturing cycle. FIGS. 4 and 5 alsoillustrate the consistency with which the extrusion process deposits thecore and shell material. The lighter areas around the circumference ofthe individual fibers represent the shell material.

[0069] 3. Warm Isostatic Pressing (WIP)

[0070]FIGS. 4 and 5 show that even in the ideal extrusion, some residualporosity occurs. Because fully dense components are desired, a ‘green’lamination process is utilized to minimize or eliminate this residualporosity from ‘green’ processing. Such a lamination process can berather complicated when eliminating green porosity in components due, inpart, to the difficulty in design and fabrication of a laminating die.

[0071] One solution is to avoid a laminating die using a warm isostaticlaminating process (WIP), which includes placing a component in apressure vessel. The vessel is sealed and heated to the requiredlaminating temperature, in this example 140° C., and an inert gas, suchas nitrogen or argon, or fluid, such as oil, is used as the compressingmedium. When pressing tubes, nozzles, thrusters and other hollowcomponents, a laminating mandrel is required to maintain the inner wallgeometry of the component. For example a water soluble mandrel materialsuch as described in U.S. Pat. No. 6,070,107 can be used. Ideally, apolymer-based mandrel material that can be poured into the hollow cavityof a green part and hardens upon drying is used.

[0072] One sample laminated was using a WIP process at 100° C. and aninitial pressure of 100 psi. The pressure in the hot state reached 125psi and the compressing medium was air. No pores were observed. In orderto prevent the compressing medium from penetrating the pores, the watersoluble mandrel is compressed in an evacuated and hermetically sealedpolymer bladder. If the compressing medium penetrates the pores, nocompression of the pores will occur.

[0073] Besides eliminating pores, WIP can be used to improve the surfacefinish. The improvement of the finish depends mainly on the stiffness ofthe polymer bladder. If the material is very soft, it conforms to eventhe slightest inconsistencies of the surface, for example the ridges inbetween individual filaments. Utilizing the correct bladder materialresulted in an improvement of the surface finish from a 50-S to a 12-Squality on the green part.

[0074] 4. Additional Co-extrusion Experiments

[0075] An 84%/16% composition was difficult to extrude since the averageparticle size of the WRe is fairly coarse. Coarse W and Re grains causethe interface to pinch out during co-extrusion resulting in adiscontinuous interface. This problem is addressed by using interfacematerial with an increased volume ratio of 31%. A 69%/31% mix proved tobe extrudable, producing a continuous WRe interface. To achieve thisresult, the milling time of the WRe powder is increased to reduce theaverage particle size. Milling time of WRe powder was increased from 24hours to 48 hours.

[0076] 5. Binder Burnout Experiments

[0077] Thermoplastic components (EEA) bind the ceramics and metalparticles and facilitate the modeling and shaping of parts. Duringbinder burnout the thermoplastic material is removed from the rest ofthe system leaving only ceramic or metal components.

[0078] The following parameters yielded the best results. The samplesare packed in a graphite powder bed within a graphite crucible. Nitrogenis the preferred environment for this process. The burnout programstretches over 48 hours ramping from room temperature to 300° C. at 50°C./hr, soaking for one hour, ramping up to 650° C. at a rate of 10°C./hr, soaking for one hour and cooling for 6 hours.

[0079] The parts are transferred to a sintering furnace after theburnout process.

[0080] 6. Hot Press, Sintering, and Sinter-HIP Experiments

[0081] The hot press conditions for the consolidation/densification ofZrC were established. Fully dense ZrC composites can be obtained byusing 10 wt % SiC as a sintering aid, and hot pressing at 1950° C., 3.3Ksi pressure. The ZrC/WRe composites fabricated were consolidated usingthese conditions. While these conditions produced good composite ZrC/WRebillets, other alternative densification/consolidation methods wereexplored because of the limitations of the uniaxial hot press indensifying more complicated three-dimensional components such as athruster nozzle. Alternative methods of consolidation such as sinteringand hot isostatic pressing (HIP) techniques were explored (Table 6) toestablish consolidation conditions for more complicatedthree-dimensional components such as bladed discs, nozzles andthrusters. TABLE 6 Run ID# Sample Temperature Atmosphere Furnace ResultHP489 Uniaxial 1950° C. N₂ 4.4 ksi Hot press Listed in Table 2. HP489Biaxial 1950° C. N₂ 4.4 ksi Hot press Listed in Table 2. HP489 UniaxialSFF 1950° C. N₂ 4.4 ksi Hot press Listed in Table 2. HP502 Uniaxial SFF1950° C. N₂ 4.4 ksi Hot press Broke before test HP509 Uni. MFCX 2x11950° C. N₂ 4.4 ksi Hot press Not Tested HP521 Uni. MFCX 2x2 1950° C. N₂4.4 ksi Hot press HP522 Uniaxial 69/31 1950° C. N₂ 4.4 ksi Hot pressHP522 Uniaxial 69/31 1950° C. N₂ 4.4 ksi Hot press Not Tested HP529 Uni.MFCX 2x2 1950° C. N₂ 4.4 ksi Hot press Not Tested

[0082] Results of the sintering experiments are shown in Table 7.Relatively high densities are achieved by sintering of both monolithicZrC and ZrC/WRe FM composite samples. All samples were placed ingraphite crucibles and heated to temperature in a graphite furnace in anargon atmosphere.

[0083] The following sintering schedule was used for the monolithicsamples:

[0084] Room Temperature to 1200° C. at 25° C./min

[0085] 1200° C.-2000° C. at 3.3° C./min

[0086] Hold at 2000° C. for 120 min

[0087] 2000° C.-1000° C. at 10° C./min

[0088] 1000° C.-Room Temperature

[0089] The ZrC/WRe sample was sintered at 2000° C., but for only onehour. Microscopic and SEM examinations indicated that the porosity inboth samples was mainly closed. Therefore, HIP processing of the sampleswill produce parts at or very close to fully dense. This resultindicates that HIP conditions were established for this materialssystem.

[0090] Using the predetermined sintering parameters, several experimentsin an Astro graphite furnace were conducted. Initial tests wereconducted on burnt-out nozzles in BN powder beds. The outcome of the BNbedded samples suggested that the very weak burnt-out structures neededeven more support, which led to a slight consolidation of the powdersupport bed. In addition, to prevent delamination and distortion, aslower heating ramp and overpressure was applied during theconsolidation process. Table 8 lists the sintering experiments. TABLE 7Conditions Results Free standing, N₂, non-wipped Poorly laminated Freestanding, N₂, wipped Delamination improved BN powder imbeddedDelamination improved further BN compacted powder bed Improveddeformation of nozzle geometry BN compacted powder bed wipped Improvedbloating problems stemming form burnout

[0091] TABLE 8 Sintering Temperature Theoretical Density % Sample (° C.)Density (g/cc) Theoretical ZrC(5%HCS SiC) 2000 6.52 5.71 88 ZrC(10%HCSSiC) 2000 6.35 5.57 88 ZrC(15%HCS SiC) 2000 6.18 5.50 89 ZrC(20%HCS SiC)2000 6.00 5.60 93 ZrC(15%PC SiC) 2000 5.83 5.44 88 ZrC(10% Zr) 2000 5.0075 ZrC(5%HCS SiC) 1950 6.52 5.13 79 ZrC(10%HCS SiC) 1950 6.35 5.01 79

[0092] A scanning electron photomicrograph of sintered ZrC/WRe slightlyrounded cell boundaries as compared to the more hexagonal cells in thehot pressed ZrC/WRe. Porosity was not evident in the hot pressed sample.

[0093] 7. Raw Materials

[0094] The following are the raw materials that were used. ProductManufacturer Article No. Quality ZrC Cerac Z-1034 −325 mesh W CeracT-1220 −325 mesh R Cerac R-1000 −325 mesh HfC Cerac H-1004 −325 mesh EEAUnion Carbide MFL 20 EEA Union Carbide MFL 1.5 EVA Union Carbide ButylOleate Union Camp

[0095] The processing of these materials is crucial for the successfulextrusion of the parts. Milling time of the metal components proved tobe of crucial importance. Milling time was increased from 24 to 48hours. In addition, torques of the different materials, measured duringthe blending of the components, must result in a ratio of 0.8 to 0.9(dividing the core material torque by the shell material torque). Toachieve this ratio the recipes of the materials were adjusted, as listedin Tables 3 and 4. The preferred ratio of core/shell material wasestablished at 69% and 31% respectively.

[0096] 8. Results of Experiments

[0097] The preferred extrusion parameters are as follows: Build Speed:4000 steps/min Extrusion Speed: 0.25 × Build Speed ExtrusionTemperature: 143° C. Slice thickness: 0.016″

[0098] In other embodiments, alternative methods of preparing FMfilaments and composite materials may be utilized. Alternativecompositions and methods, including those described in the co-pendingU.S. patent applications listed in Table 9, which are incorporated byreference herein in their entireties, are contemplated for use with thepresent invention. TABLE 9 ATTY FILING DOCKET TITLE INVENTORS DATE NO.ALIGNED COMPOSITE Anthony C. Mulligan 12/04/2001 03248.00038 STRUCTURESFOR MITIGATION Mark J. Rigali OF IMPACT DAMAGE AND Manish P. SutariaRESISTANCE TO WEAR IN Dragan Popovich DYNAMIC ENVIRONMENTS CONSOLIDATIONAND Manish P. Sutaria 12/04/2001 03248.00039 DENSIFICATION METHODS FORMark J. Rigali FIBROUS MONOLITH Ronald A. Cipriani PROCESSING Gregory J.Artz Anthony C. Mulligan COMPOSITE STRUCTURES FOR Mark J. Rigali12/04/2001 03248.00043 USE IN HIGH TEMPERATURE Manish P. SutariaAPPLICATIONS Greg E. Hilmas Anthony C. Mulligan Marlene Platero-AllRunner Mark M. Opeka COMPOSITIONS AND METHODS Mark J. Rigali12/04/2001 03248.00044 FOR PREPARING MULTIPLE- Manish P. SutariaCOMPONENT COMPOSITE Felix Gafner MATERIALS Ron Cipriani Randy EgnerRandy C. Cook MULTI-FUNCTIONAL COMPOSITE Anthony C. Mulligan 12/04/200103248.00045 STRUCTURES John Halloran Dragan Popovich Mark J. RigaliManish P. Sutaria K. Ranji Vaidyanathan Michael L. Fulcher Kenneth L.Knittel

[0099] Numerous modifications to the invention are possible to furtherimprove the processing of fibrous monolith composites. Thus,modifications and variations in the practice of the invention will beapparent to those skilled in the art upon consideration of the foregoingdetailed description of the invention. Although preferred embodimentshave been described above and illustrated in the accompanying drawings,there is no intent to limit the scope of the invention to these or otherparticular embodiments. Consequently, any such modifications andvariations are intended to be included within the scope of the followingclaims.

In the claims:
 1. A process for fabricating fibrous monolith compositescomprising: combining a first powder with a thermoplastic polymer binderand a thermoplastic plasticizer to create a uniformly suspendedcomposition, combining a second powder with a thermoplastic polymerbinder and a thermoplastic plasticizer to create a uniformly suspendedsecond composition, warm pressing the first and second uniformlysuspended compositions to create a feed rod having a first portion ofthe first composition surrounded by a second portion of the secondcomposition; and extruding the feed rod through a deposition nozzle ontoa mechanically-controlled surface to create a fibrous monolithcomposite.
 2. The process of claim 1 wherein the first and secondpowders are selected from the group consisting of metal, metal alloy,carbide, nitride, boride, oxide, phosphate and silicide.